1. Field of the Invention
The present invention relates to an electric power unit and a control method therefor and more specifically to the dead time control of a DC-DC converter.
2. Description of the Related Art
Conventionally, a DC-DC converter is known which is equipped with a low-side switch and a high-side switch. For such a DC-DC converter, to increase the efficiency it is important to optimize the dead time.
A dead time optimizing method which involves monitoring the duty and setting the dead time at which the duty is minimum as the optimum value has been proposed in “Sensorless Optimization of Dead Times in DC-DC converters with Synchronous Rectifiers” by Vahid Yousefzadeh et al., APEC, 2005, pp. 911 to 917. This method employs the feature that the efficiency of the DC-DC converter becomes maximum at the point at which the duty is minimum. The duty refers to the ratio of the on-period of the high-side switch to the sum of the on- and off-periods in one cycle.
However, the above method merely proposes optimization of dead times in the continuous conduction mode (CCM) in which the current flowing in the inductor always takes a value of more than zero (current flowing in the direction of load is defined to be positive). That is, the method does not take into consideration the discontinuous conduction mode (DCM) in which the current may become zero or negative. Therefore, the efficiency in the discontinuous conduction mode cannot be increased enough.